The present invention relates to a screen assembly for a material processing plant, more particularly, but not exclusively, to a screen assembly for a mobile crusher.
A mobile crusher is a mobile processing plant often used in the trituration of quarried materials e.g. stone. Mobile crushers are also used in the processing of aggregate from concrete or masonry waste, for example. A typical mobile crusher includes a chassis having a motor-driven crushing tool for crushing material, a hopper for feeding material to be crushed into the crushing tool, and a primary discharge conveyor for discharging crushed material which passes through the crushing tool.
The mobile crusher may include a screen assembly having one or more screens for discharging different fractions of the crushed material from the crushing tool. A screen is a device for the segregation of a material into products of different sized particles. Typically, a screen consists of a mesh mounted on a frame, which is supported on springs and fitted with an unbalanced drive, such that the screen can be vibrated to aid the passage of a predetermined size of material through the mesh.
On a mobile crusher, a screen assembly is mounted with a screen arranged beneath the primary conveyor at an angle to the horizontal. In use, crushed material is fed by the conveyor onto the upper end of the screen, such that the crushed material passes down over the vibrating screen mesh. Thus, particles of the crushed material smaller than the mesh openings are able to drop through the screen, typically on to a secondary discharge conveyor for stock piling. The particles of crushed material which do not pass through the mesh continue to travel down the screen until they are discharged, typically onto a further conveyor. The further conveyor may consist of a recirculating conveyor, for returning the oversize particles to the crushing tool, or a conveyor for discharging the oversize particles to a predetermined stock pile, for example.
The particles which pass through the screen mesh are either fed onto a second screen mounted directly below the first screen and having a smaller mesh opening size for segregation into smaller sizes, or onto a further secondary discharge conveyor, for example to be discharged on a predetermined stock pile.
Where a screen assembly is provided on a mobile crusher, it is important to be able to gain access to both the top deck and underside of the or each screen, for maintenance purposes. For example, it may be necessary to repair a damaged mesh or to change the mesh for another having a different size mesh opening. It is also desirable to be able to alter the angle at which a screen is inclined in use, to optimise efficiency of the screening process.
Unfortunately, due to the limited space available on a mobile crusher of the type described above, the or each screen is usually disposed between the primary discharge conveyor and the secondary discharge conveyor. Hence, access to the or each screen is restricted, which can lead to maintenance problems. Commonly, this is overcome by mounting a screen and an associated secondary conveyor on separate supporting frames, pivoted on the plant chassis about the lower ends of the frames, with the upper ends of each frame supported on pairs of cylinders. This arrangement enables the upper ends of each frame to be raised or lowered separately relative to one another. Access to the underside of a screen is therefore gained by lowering the secondary conveyor only, and access to the top deck of the screen is gained by lowering both the secondary conveyor and screen together.
However, this known solution is still restrictive. Moreover, there is an increasing need to reduce the weight of processing plant, such as mobile crushers, for transportation on public roads. Undesirably, the use of additional supporting frameworks for the raising or lowering of screens and conveyors, as described above, increases the already substantial weight of the mobile crusher, which is typically between 40 to 75 tonnes (metric tons).
It is an object of the invention to reduce, or substantially obviate, the disadvantages referred to above.